ON PHASE TRANSITIONS AND STRUCTURAL
SYSTEMATICS I N ALKALINE EARTH METALS
UNDER PRESSURE
H. Olijnyk, W. Holzapfel
To cite this version:
H. Olijnyk, W. Holzapfel. ON PHASE TRANSITIONS AND STRUCTURAL SYSTEMATICS
I N ALKALINE EARTH METALS UNDER PRESSURE. Journal de Physique Colloques, 1984,
45 (C8), pp.C8-157-C8-159. <10.1051/jphyscol:1984829>. <jpa-00224329>
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JOURNAL DE PHYSIQUE
Colloque C 8 , supplement au n ° l l , Tome 45, novembre 1984
page C8-157
ON PHASE TRANSITIONS AND STRUCTURAL SYSTEMATICS
IN ALKALINE EARTH
METALS UNDER PRESSURE
H. O l i j n y k and W.B. Holzapfel
Faahbereich
Physik,
liniversitaet-GH-Paderborn,
D-4790 Padevbovn,
F.R.G.
Résumé - Les structures cristallines de Mg,Ca,Sr et Ba ont été étudiées par
diffraction de rayons X en dispersion d'énergie à température ambiante et
à des pressions jusqu'à 50 GPa. Mg se transforme vers 50 GPa de hc en ce.
Des transitions structurales en ce et en cubique simple ont été observées
dans Ca respectivement à 19,5 et 32 GPa. Sr subit la transition bien
connue c f c — c e à 3,5 GPa, d'autres transitions vers 26, 35 et 46 GPa qui
conduisent finalement à la structure Ba-IV. Les expériences sur Ba ont
donné des indications sur l'existence d'une nouvelle phase, Ba-V au-dessus
de 46 GPa. Une étude des structures en relation avec les transferts s—-d
est discutée.
A b s t r a c t - The c r y s t a l s t r u c t u r e s o f Mg.Ca.Sr and Ba were studied by energy
d i s p e r s i v e X-ray d i f f r a c t i o n a t room temperature and pressures up to 50 GPa.
Mg transforms around 50 GPa from hep t o b c c . S t r u c t u r a l t r a n s i t i o n s to bec
and to a simple cubic s t r u c t u r e are observed i n Ca a t 19.5 and 32 GPa r e s p e c t i v e l y . S r shows the well known t r a n s i t i o n from fee to bcc a t 3.5 GPa
and a d d i t i o n a l phase t r a n s i t i o n s around 26, 35 and 46 GPa, which lead
u l t i m a t e l y to the Ba-IV-structure.The experiments on Ba gave some evidence
f o r the existence o f a new phase,Ba-V,above 46 GPa.Structural systematics
and t h e i r r e l a t i o n to s-=>d t r a n s f e r are discussed.
The present studies used energy d i s p e r s i v e X-ray d i f f r a c t i o n w i t h e i t h e r convent i o n a l X-ray sources / l / or w i t h synchrotron r a d i a t i o n (HASYLAB) / 2 / , a diamond
a n v i l high pressure c e l l / 3 / , a n d ruby luminescence f o r the pressure determination
w i t h the Piermarini scale / 4 / .
D i f f r a c t i o n patterns o f Mg a t various high pressures are reproduced
i n fig.1.These patterns c l e a r l y
show the occurance of a new phase
f o r Mg w i t h bcc structure.Smal1 admixtures o f the lower pressure hep
phase are i n d i c a t e d by arrows.The
l a t t i c e parameter f o r bgc-Mg a t
58 GPa is a = 2.953(3) A.The volume
change at the t r a n s i t i o n i s around
U.
F i g . 1 - D i f f r a c t i o n patterns of Mg a t
various pressures.
D i f f r a c t i o n patterns of Ba a t high
compression are shown in fig.2.Some
o f the d i f f r a c t i o n l i n e s o f Ba-IV
disappeared on increasing pressure
around 46 GPa,and also d r a s t i c
changes i n the i n t e n s i t y r a t i o s were
observed a t t h i s pressure.Typical
patterns o f Ba-IV were recorded on
decreasing pressure only below 26
GPa.These changes i n the d i f f r a c t i o n
patterns present f i r s t evidences of
a new high pressure phase,Ba-V.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984829
JOURNAL DE PHYSIQUE
C8-158
A summary o f t h e p r e s e n t l y known
phase t r a n s i t i o n s i n a l k a l i n e e a r t h
m e t a l s a t room t e m p e r a t u r e and h i g h
p r e s s u r e i s g i v e n i n t a b l e 1 toget h e r w i t h recent data f o r the d i v a l e n t r a r e e a r t h m e t a l s Eu and
Yb /5,16,17/.
The s y s t e m a t i c s o f these phase
t r a n s i t i o n s a r e more c l e a r l y seen
i f one s c a l e s t h e p r e s s u r e depend i n g a t o m i c r a d i i RA b y t h e f i x e d
i o n i c r a d i i RI a n d uses t h e s e
radius r a t i o s f o r the presentat i o n o f t h e phase t r a n s i t i o n s as
i l l u s t r a t e d i n f i g . 3 . 0 n e can not i c e t h a t Ca,Yb and S r a l l t r a n s f o r m f r o m f c c t o bcc s t r u c t u r e a t
t h e same RA/RI~l.9.Yb,Eu
and Ba
show t h e t r a n s i t i o n f r o m bcc t o
hcp s t r u c t u r e a t r a t i o 1.6 and
a l s o S r transforms a t about the
same v a l ue f r o m bcc t o Sr-111 .
20
40
60
ENERGY ( k e V )
F i g . 2 - D i f f r a c t i o n ;patterns o f Ba a t h i g h
compression.The spectrum a t 14 GPa was r e c o r d e d on u n l o a d i n g .
SC :
11
IICP
----
ORTHORH.
50
f l ~
. H C P ----
CA
Sa
: FCC
.
FCC
BCC
!9.5
---3.5
----
BCC
BCC
3a:
Eu
32
---26
SC
scc
?L?-
"(p
-.....-..-.....---3A.ly .... BA-V
HCP
----
Y B : FCC
----
----
hCC
----
HCP
SR-IV - - - - . ~ A - I V "
12
32
35
I
16
SR-III
13
:
36
----
----
Eu-Ill
46
F i r s t principle calculations
correlated the s t r u c t u r a l
sequence f c c + bcc + hcp f o r
these d i v a l e n t m e t a l s t o an e l e c t r o n i c s+d t r a n s f e r /11,12/ ,and
one can assume i n a n a l o g y t o t h e
behaviour o f the r a r e earth
metals t h a t the radius r a t i o
g i v e s a general measure f o r t h e
s j d t r a n s f e r /13/ .Furthermore,
these c a l c u l a t i o n s /12,15/ show
t h a t t h e s+d t r a n s f e r s h o u l d be
completed f o r Ba a t 50% compression,where i n f a c t % a - I V
becomes most s t a b l e . S r - V occurs
i n t h i s Ba-IV-structure a t the
same r a d i u s r a t i o and t h e r e f o r e
t h e B a - I V - s t r u c t u r e may be r e garded as t y p i c a l f o r t h e heavy
a l k a l i n e e a r t h m e t a l s a f t e r comleti ion o f s - > d t r a n s f e r . I f s + d
t r a n s f e r i n Ca i s a l s o completed
a t a r a d i u s r a t i o R~/R~:;s1.4 as
i n S r and Ba,one may e x p e c t
a n o t h e r phase t r a n s i t i o n around
70 GPa.However,Ca has no d-core
and t h i s may 1ead t o d e v i a t i o n s
from the behaviour o f the heavier
a l k a l i n e e a r t h metals a t high
compression.
-
Table 1: Phase t r a n s i t i o n s and
t r a n s i t i o n pressures.
Be: Ref .6
Eu,Yb:Ref.5,16,17
Ca,Sr:Ref . 8
Ba:Ref.9,8
------------------hcp
6avIBal~lhcp(
. .
.
1 bce Ifee (
bcc 1 rcc I
bcc
-
1
...
-
-
I f the analogy between heavy
a l k a l i n e e a r t h metals and Eu and
Yb would continue t o h o l d a t
h i g h e r pressures ,the l a t t e r two
elements should transform t o the
Ba-IV-structure around 35 and 55
GPa r e s p e c t i v e l y . A c t u a l 1 y Eu-111,
which i s s t a b l e above 32GPa and
up t o a t l e a s t 45 GPa /5/ i s
d i f f e r e n t from the Ba-IV-structure.
2,.0
- . - .
2:5
. . .
Ratio of atomic to ionic radii: RA /RI
I
Obviously, the 1ack o f d-bands
near t h e Fermi-level i n Mg
explains e a s i l y w i t h i n t h i s
framework a l s o the d i f f e r e n t
s t r u c t u r a l behaviour o f t h i s
l i g h t d i v a l e n t metal 1141.
F i g . 3 - S t r u c t u r a l systematics.
The values f o r RI were taken from r e f . 10
Z denotes the atomic number.
This work was supported i n p a r t by HASYLAB and the Sundesministerium f u r Forschung
und Technologie.
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